As electrical distribution grids continue to grow in both size and complexity, the need to control and mitigate inevitable faults within these systems is an ongoing issue. Fault Current Limiters (FCL) are devices for protection of components and parts of electrical distribution networks. The role of a FCL is to act as a fast switching element, much faster than existing circuit breakers. The FCL is introduced into the power distribution network in series with a load. If the load shorts, a high current would be generated; the high current has the capacity to damage other devices in the network. The FCL is activated once the load shorts and the high current is initiated. The FCL, once activated, acts to suppress the large current transient and thus protect other devices. Conventional fault current limiting technology leaves considerable room for improvement in both cost and timeliness of the responsive to faults.
Depending on their architecture, superconducting FCLs fall into one of two main categories, resistive or inductive. Inductive FCLs themselves come in many designs; the simplest is a transformer with the primary in series with the load and a closed superconducting ring as the secondary. In un-faulted operation, there is no resistance in the secondary and so the inductance of the device is low. A fault current quenches the superconductor (SC), the secondary becomes resistive and the inductance of the whole device rises, increasing the impedance of the primary winding and limiting the fault. While many variants of the inductive type limiters exist, the main advantage of this design is that there is no heat ingress through current leads into the SC, and so the cryogenic power load may be lower. However, inductive FCL typically require substantially more superconductor wire than resistive type, leading to comparatively larger AC losses as well as initial capital costs.
Conventional resistive type FCL, on the other hand, operate based on the principle that the load current passes directly through the superconductor (SC) element, and when a high fault current begins, the superconductor quenches: it becomes a normal conductor as the temperature and thus the resistance rises sharply and quickly. This extra resistance in the system reduces the fault current from what it would otherwise be (the prospective fault current) but correspondingly, creates a large increase in temperature that must be removed. If the superconductor operates in a temperature range that falls within convenient liquid cryogen temperature ranges, the excess energy can quickly be removed by exposure to the liquid cryogen. However, liquid cryogen requires substantial engineering and handling costs due to, among other things, generated pressures during cooling of the superconductor. In contrast, conduction cooling has been viewed as impractical for conventional systems due to the relatively low energy removal rates compared to liquid cryogen immersion.
A resistive FCL can be either DC or AC. If it is AC, then there will be a steady power dissipation from AC losses (superconducting hysteresis losses and possibly eddy current losses) which must be removed by the cryogenic system. An AC FCL is usually made from wire wound non-inductively; otherwise the inductance of the device would introduce an unwanted, parasitic impedance.
Rapid response and recovery time are essential elements to FCL viability. In power utility applications, it would be desirable to have this recovery time as small as possible, within a second if possible. For some applications, recovery within minutes is acceptable, and in general there would be a tradeoff between shorter recovery time and acceptable system cost. Some proposed FCL based on Yttrium barium copper oxide (YBCO) superconductors and liquid cryogen cooling claim to offer rapid recovery time, however, the capital cost is very high. Potentially much lower cost systems based on Magnesium diboride (MgB2) with the use of cryo-cooling systems have also been proposed, but as designed, these will have long recovery times relative to the limited fault (typically hours). The system disclosed in this work describes a low cost system which does not need liquid cryogen (instead using conduction-cryocooling), has low operating AC loss, and have fast recovery.